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DAY OF'YEAR DAY OFYEAR
Figure 2.--Graphs of water regimes, the course of xylem growth and development, vertical resin duct density in the
current annual ring, and resin yield from standard bark wounds in 1986 (A, B, C, D) and 1987 (E, F, G, H).
Daily soil water storage (solid line) and cumulative daily water deficits (dotted line), (A, E). Tree bole cambial
growth and development expressed as the number of tracheids formed, time of transition from earlywood to
latewood, and the total amount of earlywood and latewood formed (B, F). Vertical resin duct densities in the
current annual ring (C, G). Oleoresin yield over 24-hour periods (D, H). Vertical bars are standard errors, n=l 1
for 1986, n--13 for 1987. Water regimes, growth and development, and resin yields differ dramatically between
years. Resin yields for 1987 closely approximate the seasonal changes in resistance suggested in the conceptual
model shown in Figure 3 for resistance to southern pine beetle attack in years that have soil water regimes 203
similar to that of the long-term average. Severely dry conditions in the summer of 1986 apparently not only
limited carbon partitioning not only to growth, but also to secondary metabolism; e.g., oleoresin synthesis.
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